Spectacular progress in the brightness, lumen efficacy and affordability of solid state light sources such as light-emitting-diodes (LEDs) enables new lighting applications that are no longer restricted to niche markets. LEDs offer several advantages over traditional light sources, such as long lifetime, superior efficiency, low operating voltage, design flexibility, more pure spectral colors, fast response times.
For these and other reasons, LEDs are becoming more and more suited for making illumination devices, like color variable luminaires, spotlights, LCD backlighting, architectural lighting, stage lighting, etc.
For many solid state light sources, such as organic LEDs (OLEDs), most efficient operation is achieved with DC biasing at the optimal efficacy point. For higher brightness levels, biasing above this point is required. Brightness can be controlled with Amplitude Modulation (AM) or with Pulse Width Modulation (PWM) where the LED is duty-cycled. Often PWM is applied for low losses in the driver, and allowing freedom of system supply voltage.
In some applications, light emitting devices covering a larger surface are required. Many LEDs are then connected in parallel on a substrate, or, in case of OLED, one or several OLED tiles covering a large surface are formed. When driving such light emitting devices, the occurrence of short circuits become a problem, as the entire system is short circuited and no light is emitted in case of a large short-circuit current. The current also results in unwanted power dissipation and heat generation.
Conventionally, this problem has been addressed by for example selecting OLED tiles with a sufficiently low chance for defects. Also burn-in procedures and reverse biasing is used to reduce and/or heal the effect of shorts. However, the occurrence of shorts in OLED for lighting applications may be difficult to avoid completely, due to the large area needed and the strive for simple low-cost technology with sufficient yield.
U.S. Pat. No. 7,052,473 proposes AC drive of LEDs, with two LEDs connected anti-parallel in series with a capacitor. During every current cycle, a limited amount of charge is forced through the LED resulting in a light flash. This charge is stored on the capacitor, which stops the current. The charge is available for the next cycle to be used again. The anti-parallel diode ensures back-flow of the charge. In this case full double-phase operation is assured.